Ferritic, austenitic, martensitic stainless steel

ABSTRACT

Rollable and weldable stainless steel with a high degree of tension and toughness within a wide range of temperatures both in a hardened and a tough-hardened condition comprising 5-15 percent by weight of ferrite, 10-40 percent by weight of austenite and the remainder tempered martensite.

United States Patent Morsing 1451 Mar. 21,1972

[54] FERRITIC,AUSTENITIC, [56] References Cited MARTENSITIC STAIN LESSSTEEL UNITED STATES PATENTS [721 Invent f Avesm 2,602,737 7/1952 Bindel....75/l28N e 2,851,384 9/1958 Waxweiler... ....75/I28N [73] Assignee:Avesta Jernverks Aktiebolag, Avesta, 3,102,025 8/1963 Wilcox "-75/l28NSweden 3,253,966 5/1966 Malagari... ...75/12s N 3,276,864 10/1966Loginow ..75/l28N 22 F1led: 061.13, 1969 3,306,736 2/1967 Rundell..75/l28N 21 i 51196.; 871,396

Related US. Application Data Continuation-impart of Ser. No. 856,872,July 3, 1969, which is a continuation of Sen No. 558,998, June 20, 1966,abandoned.

Primary Examiner-Hyland Bizot Attorney-Brumbaugh, Graves. Donohue &Raymond ABSTRACT 3 Claims, 1 Drawing Figure atented arch 21, 19723,650,709

NTOR

HAN 6U5TAF NURSING 1 LA R i L his ATTORNEYS FERRHTHC, AUSTENITHC,MARTENSITIC STAINLESS STEEL This is a continuation-in-part of mycopending application Ser. No. 856,872 filed July 3, 1969 which is inturn a continua- Uliimate strength kgJmm. Hardness Brincll The SwedishStandards Association However, the weldability of steels of this type isnot as good, since they are sensitive to grain growth. In welding,the'heatmin 60 max 260 non of my apphcann. N 558998 filed June 1966actuated zone becomes coarse-grained, a circumstance which nowabandoned. This invention relates to rollable and weldarenders thesteels brittle ble stainless steel with a high tension and toughnesswithin a The Steel accordin the resent invention is charac wide range oftemperature both in a hardened and a toughterizedin that com rgises phardened condition and to products produced from the steel. p Among theproducts welded constructions can especially be o 020 0 040 Emmy 0 g o040 Carbon mentioned, e.g., pressure vessels where it is desirable togummy silicon reduce the thickness of the material by using a materialwith a 0.2 2.0 preferably 0.6 1.0 Manganese high strength. The presentinvention relates further to a stainpekrab'y 'f less sheet iron materialwith a high strength, good corrosion 3 I g g; gtzl resistance anda lowcoeff cient of thermal expansion for use 0'02 preferably Nmogcn as Pressplatens. m h proces,s of pres.smg wanboards and the balance being ironand the contents of the alloying materilamlnates (formica). Finally,wire and strip products may also als mutually adapted in Such a way thatthe chromium memiond for example as intended to be used as equivalentplus the nickel equivalent are between 24.5 and z mate.r.lalS P of i 226.2 and 1.4 times the chromium equivalent minus the nickel usteilltlcstainless steels are mainly used for welded constructions of steelresistant to corrosion. These steels are nor- 2221; 3 :255???2223;"; fsrs i g g fi3: 2:52: mall known as 18-8 18 ercent Cr, 8 ercent Ni 1 -10(18 gercem Cr, 10 peremgny pacer) 10 equivalent percent Ni +0.5 percentMn 30 X (percent C pace", Ni plus about percent Mo), 2542 (25 percentpercent N); as a result after the hardening the structure con- Cr 12percfim Ni) 25 20 (25 percent Cr 20 percent Ni) etc. talns martensite,ferrite and austenite. From the analytic and They are characterized by avery good toughness, go structural point of view the steel may bedefined as a transfortimy and Very good weldabmty. In comparison toumallowd mation form between an austenltlc and a ferrlte-martensitlc andlow-alloyed steel of a ferrite-carbide structure (such as Steel Vferrite plus perlite, bainite, tempered martensite) they have a ThePhase FomPosmon P the alloys fi to the low strength. The yield limit(0.2-limit) is particularly low, prese m q 15 shown m graphlcal form mthe moon" in general below 25 kgJmmz' Owing to this circumstance a iii tl i e ii izgi fii the chromium equivalent is given as the abconstruction exposed to mechanical stresses, e. a ressure vessel ofaustenitic stainless steel, often has an u necozomical Sciss? and thenlckel q a nt as he Otdtnate. The alloys acthickness. In order to reducethe stainless steel portion it is cordmg the P mvemlon he the fiashedarea therefore sometimes necessary to make use of a stainless lin- ABCD,bemg Wlthm the three-Phase areal austemte P ing Combined with certainhighustrength un a1]oyed or 1 1 tensite plus ferrite. Owing to thiscondition the structure of the loyed steels or to so-called compoundsteel, a method which in alloys will after rolling and hardening atabove C and both cases leads to certain technical difficulties, e.g.,welding pering a! 5 0-630 C., contain 5-15 percent f r problemg In piteof their low strength, however, {he 0 1H0 percent austenite, and theremainder tempered maraustenitic steels are very widely utilized owingto their great tensltei toughness, good ductility and particularlybecause they are The Steel is heat'n'ealed y means of hardening at moreeasy to weld. than 900 C. and subsequent tempering within a temperatureIn cases when greater strength is required and the demands rangeof500630 C. At the hardening temperature the strucon corrosionresistance are moderate certain stainless steels ture consists ofaustenite plus 5 to 15 percent ferrite. In the have up to now been used,their high strength being obtained process of cooling to roomtemperature or below no transforby means of a high carbon content. Thisrefers to so-called mation of the ferrite takes place, while theaustenite is transmartensite steels which reach their high strength inconnecformed into martensite. However, a few per cent of the tion withhardening and tempering. In these steels chromium austenite remainuntransformed. is the main alloying element. As an example somestainless By heating the hardened steel up to 500-630 C. a tempersteelsstandardized in Sweden having 13-17 percent of ing of the martensite andan equalization of the hardening chromi m m y e m n ione stresses isobtained, which is normal for toughened steel, and

Yield Ultimate Condi- Carbon Chromium Nickel limit strength Hardnesstion (percent) (percent) (percent) kg/ntnf kglmrri Brinell 4 0.ls0.25...min 12.0.. min 70 90-105....... 270-320 -3 0.30-0.40... min 13.0.. min150...... *455-625 -3 max 0.25.... min l6........2.5-3.0..... min65...... 85l00....... 250-300 *Vickers. The figures refer to thecondition according to the standards. ***The Swedish StandardsAssociation.

The drawback re ardin these steels is the hi h carb c ntent which means%hat th e steels grow sensitife to cr i ks in toughiless of the Stee} 'fHowever what dlsEmguishes this type of steel is that in the process oftempering welding slmie the whlch 15 Created m the heatac' within thisrange of temperature austenite is also created tuated zone is hard andbrittle. In order to avoid the formation wh.ch stable also at low temeramres This creation of of cracks in these steels it is necessary topre-heat them in the bl I it incr Ses with anpincreasea tem in {em ff ofweidling p Fi f the g tui e ii; :2 aTaout 230 C If heated up to high rIBEIPG'JZUICS ere are aso errlte austenitic stain ess stees aving apomparativglyiigh yield limit such as, for example, the folowmg Stan at:21 2324 ing the temperature and the length of time of the temperingCarbon m x .1 different levels of strength having yield limits varyingbetween 2:1 50 and 80 kg./mm. are obtained. A longer tempering timeMolybdenum 1% LL13 75 reacts in the same direction as an increase in thetempering Yield limit kgJmm. min 42 temperature.

In spite of the low carbon content the steel has a high strength. Itdiffers from the martensite steels having a high carbon content by beingeasy to weld. The martensite which is created in the heat-actuated zoneis tough owing to the low carbon content of the steel. Consequently,this zone is not embrittled which it is as regards the chromium steelswith a high carbon content. The austenite which is contained in thesteel also contributes to the toughness of the heat-actuated zone.Consequently, the steel is not sensitive to cracks in the process ofwelding and can be welded without pre-heating. In order to obtainmaximum toughness characteristics it is suitable to anneal after thewelding within a temperature range of 500-630 C. This produces anequalization of the welding stresses and a tempering of the martensitein the hardening zone. The annealing temperature that should be chosendepends on the temperature used in the tempering in connection with thetough hardening of the steel.

A characteristic feature of this steel is also that it has a certaincontent of ferrite. As regards hardable steels having a high content ofcarbon the problem has been how to adapt the analysis in such a way thatferrite is avoided. It is a well-known fact that ferrite in themartensitic steels with a high content of carbon lowers the toughness,which is the reason why the content of ferrite-forming substances mustbe maintained at a low rate when high toughness is required. If, on theother hand, the content of carbon is low, it has been proved that a goodtoughness of martensitic steels is obtained in spite of the existence offerrite in the structure.

Thanks to this it has been possible to keep the content of chromium highat about 16 percent, and it has also been possible to add about 1percent molybdenum. This means a considerably higher resistance tocorrosion as compared with the results obtained with the previouslyknown ferrite-free steels, whose content of chromium must be limited toabout 14 percent if there is no molybdenum, and to about 13 percent ifabout 1 percent molybdenum is desired. Since the steel according to thepresent invention is hardenable it differs from the ferrite containingsteels according to the French Patent specification No. 803.361 having0.020-about 0.300 percent C, 1.3-3.0 percent Mn, 1.5-6.5 percent Ni,16.0-23.0 percent Cr and possibly up to about 2.5 percent Cu or 3percent M or 3 percent W. In these steels with their composition soadapted that two phases, ferrite plus austenite are obtained, the objectof using ferrite is to reduce the sensitivity to inter-granularcorrosion characteristic for austenitic steels having a high content ofcarbon.

The high toughness of the steel is also made obvious by the circumstancethat the curve for impact work at various temperatures is rectilinearwithout any marked transition from a high to a low value and with atransition temperature below l96 C. As the transition temperature atimpact tests it is usual to indicate the temperature at which the curvefor impact work versus the temperature for an impact test specimen withan area of 10 X 10 mm?, a V-indication 2 mm. deep and a support distanceof 45 mm. passes the value 2.8 kg. For welded constructions, especiallyfor pressure vessels, it is desirable to use material with a lowtransition temperature; it is often prescribed that the transitiontemperature shall lie below 0 C. or 20 C. At 196 C. the present steelshows an impact work of about kg., which is considerably above 2.8 kg.,a value which is regarded as a criterion of the transition temperature.

The steel is also characterized in that the high temperature yield limitup to 300-400 C. is only slightly less than the yield limit at roomtemperature.

The steel has a good resistance to irradiation with neutrons in anuclear reactor, and it has even after being irradiated to neutron dosesof 3X10 neutrons/cm. 1 mev.) a transition temperature for impact testsbelow 80 C.

The steel has a low coefficient of expansion. At 20100 C. it is 12 X itis thus of the same magnitude as unalloyed steel and considerably lowerthan the figures for austenitic steel.

EXAMPLE 1 5 Sheet iron havin a thickness of 14 mm. was roduced from g asteel having the following composition:

Percent 10 P 0.011 5 0.005 Cr 16.0 Ni 4.7 Mo 1.02 N 0.03s

the remainder consisted of iron and unintentional impurities.

The sheet iron was heated up to 900 C., which temperature was maintainedfor 1 hour; it was then hardened in air and tempered at 600 C., thelatter temperature being maintained for 6 hours. After testing the sheetiron heat-treated in this manner the following strength values wereobtained:

Yield limit, 0.2-point" 60 kgJmm. Ultimate Strength 91 kgJmm.Elongation, S X d 20% Impact work, 20 kv. 7.2 kg.

- Hardness, Brinell 278 At a higher temperature the following strengthvalues were obtained:

100C. 200c. 300C. 400C. 500 c.

Yield limit,

kgJmm. 60 so 60 s9 51 Ultimate strength,

kgJmm. 84 so 79 13 63 Elongation,

s x d 2o l8 l6 14 14 Impact work at different temperatures:

Testing temperatures l96C. 74C. 20 c. 0C. +20" c.

Impact work,

kv., kg. 5.3 6.3 6.7 7.1 1.2

The sheet iron was welded by means of coated electrodes having a similarcomposition. No cracks were noticed either in the weldings or in thezone actuated by the welding heat.

Example 2 70 mm sheet iron of the same charge as that used in Example lwas hardened in air after being heated up to 900 C. for 2 hours andtempered at 600 C. for 6 hours. The following strength values wereobtained:

Yield limit 64 kgJmm. Ultimate strength 92 kgJmm. Elongation, 5 X d 18%Impact work, kv. 7.3 kg.

EXAMPLE 3 A forging round with a diameter of 170 mm. and 250 mm. in

length was heat-treated by heating up to 900 C. for 4 hours subsequentcooling in air and tempering at 575 C. for 6 hours.

The composition of the steel in percent by weight was:

C Si Mn P S Cr Ni Mo N 0.035 0.30 1.00 0.035 0.025 16.5 4.7 1.08 0.030

V 7 'llhe f ollowing strength values were obtained:

Contraction,

Impact work at different temperatures:

Testing temperature -l96 C. -74 C. -20 C. C. +20 C. Impact work,

kv., kg. 6.2 11 l2 13 14 EXAMPLE 4 From 70-mm. sheet iron of the samecharge as that in Example 1 tensile and and impact test specimen weretaken which were emitted in a nuclear reactor at 240260 C.

Strength tests prior to and after the irradiation resulted in thefollowing values:

Quantity of Irradiation Yield Ultimate Elonganeutrons/cm. Temperaturelimit strength tion 7:

1 mev.) kgJmmFkg/mm. Unirradiated 66 88 18 5-10 265 C. 90 102 17Transition Temperature for Impact Tests Impact test specimen 3 X 3 mm. T0.17 is the temperature at which impact work 0.17 kg. is obtained andcorresponds approximately to the temperature T 2.8, at which impact work2.8 kg. is obtained for a normal test specimen.

Quantity of Irradiation neutrons/cm. Temperature T0,l7 HV 1 mev.)

Unirradiated l92 293 5.000 X 10" 265C. l37 332 1 claim:

1. Welded constructions of stainless steel characterized in that boththe basic metal and the weld metal comprise steel of a tough hardenedtype, containing:

Percent 0020-0040 Carbon 0.2-1.0 Silicon 0.22.0 Manganese 15-18 Chromium4-6 Nickel 0.2 Molybdenum 0020-0080 Nitrogen and the balance iron withthe usual impurities, the contents of the alloying materials beingmutually adapted such that the chromium equivalent plus the nickelequivalent are between 24.5 and 26.2 and 1.4 times the chromiumequivalent and the nickel equivalent are between 16.5 and 19.0, wherethe chromium equivalent is percent Cr percent Si percent Mo and thenickel equivalent percent Ni 0.5 percent Mn 30 (percent C percent N),said stainless steel having been hardened by heating up to at least900C. to obt'ain'a structure of austenite and ferrite only and thenrapidly cooled to a low temperature so that most of the austenite wastransformed into martensite, and thereafter tempered by heating up to500-630 C. to obtain a structure comprising 5-15 percent by weight offerrite, 1040 percent by weight of austenite and the remainder temperedmartensite.

2. Steel according to claim 1 wherein the steel contains:

Percent 0030-0040 Carbon 0.3-0.5 Silicon 0.6-1.0 Manganese 16-17Chromium 4.5-5.5 Nickel 0.8-1.3 Molybdenum 0.0200.050 Nitrogen 3. Weldedconstructions produced from the steel according to claim 1, intended tobe exposed to neutron irradiation.

zg gg UNITED STATES PATENT OFFICE CERTIFlC-ATE 0F CORRECTION Patent: No.3,650,709 Dated March 21, 1972 lnve Lars Johan Gustaf Morsing It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5, line 32, "5,000" should be -5 ,o0--; Column 6, line 13,"equivalent and the" should be -equivalent minus the--. 1

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR., ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,650,709 Dated March 21, 1972 InVentOr() Lars Johan Gustaf MorsingIt is certified that errot appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 5, line 32, "5,000" should be 5,o0--; Column 6, line 13,"equivalent and the" should be ---equivalent minus the--.

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,J'R. ROERI GOTTSCHALK Attesting Officer Commissionerof Patents

2. Steel according to claim 1 wherein the steel contains: Percent0.030-0.040Carbon0.3-0.5Silicon0.6-1.0Manganese16-17Chromium4.5-5.5Nickel0.8-1.3Molybdenum0.020-0.050Nitrogen3. Welded constructions produced from the steel according to claim 1,intended to be exposed to neutron irradiation.